Pneumatic tire

ABSTRACT

The present invention is directed to a pneumatic tire having a tread comprising a vulcanizable rubber composition comprising, based on 100 parts by weight of elastomer (phr),
         (A) from about 60 to about 40 phr of a solution polymerized styrene-butadiene rubber having a glass transition temperature (Tg) ranging from −85° C. to −50° C.;   (B) from about 40 to about 60 phr of natural rubber; and   (C) from about 5 to 40 phr of a process oil;   (D) optionally, from about 5 to 15 phr of a hydrocarbon traction resin   (E) from 60 to 90 phr of silica;
 
wherein the weight ratio of silica to the sum of the amounts of oil and traction resin is greater than 2.2.

BACKGROUND OF THE INVENTION

It is highly desirable for tires to have good wet skid resistance, lowrolling resistance, and good wear characteristics. It has traditionallybeen very difficult to improve a tire's wear characteristics withoutsacrificing its wet skid resistance and traction characteristics. Theseproperties depend, to a great extent, on the dynamic viscoelasticproperties of the rubbers utilized in making the tire.

In order to reduce the rolling resistance and to improve the treadwearcharacteristics of tires, rubbers having a high rebound havetraditionally been utilized in making tire tread rubber compounds. Onthe other hand, in order to increase the wet skid resistance of a tire,rubbers which undergo a large energy loss have generally been utilizedin the tire's tread. In order to balance these two viscoelasticallyinconsistent properties, mixtures of various types of synthetic andnatural rubber are normally utilized in tire treads.

Tires are sometimes desired with treads for promoting traction on snowysurfaces. Various rubber compositions may be proposed for tire treads.Here, the challenge is to reduce the cured stiffness of such treadrubber compositions, as indicated by having a lower storage modulus G′at −20° C., when the tread is intended to be used for low temperaturewinter conditions, particularly for vehicular snow driving.

It is considered that significant challenges are presented for providingsuch tire tread rubber compositions for maintaining both their wettraction while promoting low temperature (e.g. winter) performance

SUMMARY OF THE INVENTION

The present invention is directed to a pneumatic tire having a treadcomprising a vulcanizable rubber composition comprising, based on 100parts by weight of elastomer (phr),

-   (A) from about 60 to about 40 phr of a solution polymerized    styrene-butadiene rubber having a glass transition temperature (Tg)    ranging from −85° C. to −50° C.;-   (B) from about 40 to about 60 phr of natural rubber; and-   (C) from about 5 to 40 phr of a process oil;-   (D) optionally, from about 5 to 15 phr of a hydrocarbon traction    resin-   (E) from 60 to 90 phr of silica;    wherein the weight ratio of silica to the sum of oil and traction    resin is greater than 2.2.

The invention is further directed to a method of making a tire.

DESCRIPTION OF THE INVENTION

There is disclosed a pneumatic tire having a tread comprising avulcanizable rubber composition comprising, based on 100 parts by weightof elastomer (phr),

-   (A) from about 60 to about 40 phr of a solution polymerized    styrene-butadiene rubber having a glass transition temperature (Tg)    ranging from −85° C. to −50° C.;-   (B) from about 40 to about 60 phr of natural rubber; and-   (C) from about 5 to 40 phr of a process oil;-   (D) optionally, from about 5 to 15 phr of a hydrocarbon traction    resin-   (E) from 60 to 90 phr of silica;    wherein the weight ratio of silica to the sum of oil and traction    resin is greater than 2.2.

There is further disclosed a method of making a tire.

The rubber composition includes from 40 to 60 phr of a styrene-butadienerubber having a glass transition temperature (Tg) ranging from −85° C.to −50° C. The styrene-butadiene rubber may be functionalized withvarious functional groups, or the styrene-butadiene rubber may benon-functionalized. In on embodiment the styrene-butadiene rubber isfunctionalized with an alkoxysilane group and at least one of a primaryamine group and thiol group. In one embodiment, the styrene-butadienerubber is obtained by copolymerizing styrene and butadiene, andcharacterized in that the styrene-butadiene rubber has a primary aminogroup and/or thiol group and an alkoxysilyl group which are bonded tothe polymer chain. In one embodiment, the alkoxysilyl group is anethoxysilyl group. In one embodiment, the styrene-butadiene rubber isnot functionalized.

The primary amino group and/or thiol group may be bonded to any of apolymerization initiating terminal, a polymerization terminatingterminal, a main chain of the styrene-butadiene rubber and a side chain,as long as it is bonded to the styrene-butadiene rubber chain. However,the primary amino group and/or thiol group is preferably introduced tothe polymerization initiating terminal or the polymerization terminatingterminal, in that the disappearance of energy at a polymer terminal isinhibited to improve hysteresis loss characteristics.

Further, the content of the alkoxysilyl group bonded to the polymerchain of the (co)polymer rubber is preferably from 0.5 to 200 mmol/kg ofstyrene-butadiene rubber. The content is more preferably from 1 to 100mmol/kg of styrene-butadiene rubber, and particularly preferably from 2to 50 mmol/kg of styrene-butadiene rubber.

The alkoxysilyl group may be bonded to any of the polymerizationinitiating terminal, the polymerization terminating terminal, the mainchain of the (co)polymer and the side chain, as long as it is bonded tothe (co)polymer chain. However, the alkoxysilyl group is preferablyintroduced to the polymerization initiating terminal or thepolymerization terminating terminal, in that the disappearance of energyis inhibited from the (co)polymer terminal to be able to improvehysteresis loss characteristics.

The styrene-butadiene rubber can be produced by polymerizing styrene andbutadiene in a hydrocarbon solvent by anionic polymerization using anorganic alkali metal and/or an organic alkali earth metal as aninitiator, adding a terminating agent compound having a primary aminogroup protected with a protective group and/or a thiol group protectedwith a protecting group and an alkoxysilyl group to react it with aliving polymer chain terminal at the time when the polymerization hassubstantially completed, and then conducting deblocking, for example, byhydrolysis or other appropriate procedure. In one embodiment, thestyrene-butadiene rubber can be produced as disclosed in U.S. Pat. No.7,342,070. In another embodiment, the styrene-butadiene rubber can beproduced as disclosed in WO 2007/047943.

In one embodiment, and as taught in U.S. Pat. No. 7,342,070, thestyrene-butadiene rubber is of the formula (I) or (II)

wherein P is a (co)polymer chain of a conjugated diolefin or aconjugated diolefin and an aromatic vinyl compound, R¹ is an alkylenegroup having 1 to 12 carbon atoms, R² and R³ are each independently analkyl group having 1 to 20 carbon atoms, an allyl group or an arylgroup, n is an integer of 1 or 2, m is an integer of 1 or 2, and k is aninteger of 1 or 2, with the proviso that n+m+k is an integer of 3 or 4,

wherein P, R¹, R² and R³ have the same definitions as give for theabove-mentioned formula I, j is an integer of 1 to 3, and h is aninteger of 1 to 3, with the provision that j+h is an integer of 2 to 4.

The terminating agent compound having a protected primary amino groupand an alkoxysilyl group may be any of various compounds as are known inthe art. In one embodiment, the compound having a protected primaryamino group and an alkoxysilyl group may include, for example,N,N-bis(trimethylsilyl)aminopropylmethyldimethoxysilane,1-trimethylsilyl-2,2-dimethoxy-1-aza-2-silacyclopentane,N,N-bis(trimethylsilyl)aminopropyltrimethoxysilane,N,N-bis(trimethylsilyl)aminopropyltriethoxysilane,N,N-bis(trimethylsilyl)aminopropylmethyldiethoxysilane,N,N-bis(trimethylsilyl)aminoethyltrimethoxysilane,N,N-bis(trimethylsilyl)-aminoethyltriethoxysilne,N,N-bis(trimethylsilyl)aminoethylmethyldimethoxysilane,N,N-bis(trimethylsilyl)aminoethylmethyldiethoxysilane, etc., andpreferred are 1-trimethylsilyl-2,2-dimethoxy-1-aza-2-silacyclopentane,N,N-bis(trimethylsilyl) aminopropylmethyldimethoxysilane andN,N-bis(trimethylsilyl)aminopropylmethyldiethoxysilane. In oneembodiment, the compound having a protected primary amino group and analkoxysilyl group is N,N-bis(trimethylsilyl)aminopropyltriethoxysilane.

In one embodiment, the compound having a protected primary amino groupand an alkoxysilyl group may be any compound of formula III

RN—(CH₂)_(x)Si(OR′)_(3,)   III

wherein R in combination with the nitrogen (N) atom is a protected aminegroup which upon appropriate post-treatment yields a primary amine, R′represents a group having 1 to 18 carbon atoms selected from an alkyl, acycloalkyl, an allyl, or an aryl; and X is an integer from 1 to 20. Inone embodiment, at least one R′ group is an ethyl radical. Byappropriate post-treatment to yield a primary amine, it is meant thatsubsequent to reaction of the living polymer with the compound having aprotected primary amino group and an alkoxysilyl group, the protectinggroups are removed. For example, in the case of bis(trialkylsilyl)protecting group as inN,N-bis(trimethylsilyl)aminopropyltriethoxysilane, hydrolysis is used toremove the trialkylsilyl groups and leave the primary amine.

In one embodiment, the rubber composition includes from about 40 toabout 60 phr of styrene-butadiene rubber functionalized with analkoxysilane group and a primary amine group or thiol group.

Suitable styrene-butadiene rubbers functionalized with an alkoxysilanegroup and a primary amine group are available commercially, such as HPR340 from Japan Synthetic Rubber (JSR).

In one embodiment, the solution polymerized styrene-butadiene rubber isas disclosed in WO 2007/047943 and is functionalized with analkoxysilane group and a thiol, and comprises the reaction product of aliving anionic polymer and a silane-sulfide modifier represented by theformula VII

(R⁴O)_(x)R⁴ _(y)Si—R⁵—S—SiR⁴ ₃   VII

wherein Si is silicon; S is sulfur; O is oxygen; x is an integerselected from 1, 2 and 3; y is an integer selected from 0, 1, and 2;x+y=3; R⁴ is the same or different and is (C₁-C₁₆) alkyl; and R′ isaryl, and alkyl aryl, or (C₁-C₁₆) alkyl. In one embodiment, R⁵ is a(C₁-C₁₆) alkyl. In one embodiment, each R⁴ group is the same ordifferent, and each is independently a C₁-C₅ alkyl, and R⁵ is C₁-C₅alkyl.

The solution polymerized styrene-butadiene rubber has a glass transitiontemperature in a range from −85° C. to −50° C. A reference to glasstransition temperature, or Tg, of an elastomer or elastomer composition,where referred to herein, represents the glass transition temperature(s)of the respective elastomer or elastomer composition in its uncuredstate or possibly a cured state in a case of an elastomer composition. ATg can be suitably determined as a peak midpoint by a differentialscanning calorimeter (DSC) at a temperature rate of increase of 10° C.per minute, for example according to ASTM D7426 or equivalent.

Suitable styrene-butadiene rubbers functionalized with an alkoxysilanegroup and a thiol group are available commercially, such as Sprintan SLR3402 from Trinseo.

Another component of the rubber composition is from about 40 to about 60phr of natural rubber.

The rubber composition may include 10 to 40 phr of a processing oil.Processing oil may be included in the rubber composition as extendingoil typically used to extend elastomers. Processing oil may also beincluded in the rubber composition by addition of the oil directlyduring rubber compounding. The processing oil used may include bothextending oil present in the elastomers, and process oil added duringcompounding. Suitable process oils include various oils as are known inthe art, including aromatic, paraffinic, naphthenic, and low PCA oils,such as MES, TDAE, and heavy naphthenic oils, vegetable oils such assunflower, soybean, and safflower oils, and monoesters of fatty acidsselected from the group consisting of alkyl oleates, alkyl stearates,alkyl linoleates, and alkyl palmitates.

In one embodiment, the rubber composition includes a low PCA oil.Suitable low PCA oils include but are not limited to mild extractionsolvates (MES), treated distillate aromatic extracts (TDAE), and heavynaphthenic oils as are known in the art; see for example U.S. Pat. Nos.5,504,135; 6,103,808; 6,399,697; 6,410,816; 6,248,929; 6,146,520; U.S.Published Applications 2001/00023307; 2002/0000280; 2002/0045697;2001/0007049; EP0839891; JP2002097369; ES2122917. Generally, suitablelow PCA oils include those having a glass transition temperature Tg in arange of from about −40° C. to about −80° C. MES oils generally have aTg in a range of from about −57° C. to about −63° C. TDAE oils generallyhave a Tg in a range of from about −44° C. to about −50° C. Heavynaphthenic oils generally have a Tg in a range of from about −42° C. toabout −48° C. A suitable measurement for Tg of TDAE oils is DSCaccording to ASTM E1356, or equivalent.

Suitable low PCA oils include those having a polycyclic aromatic contentof less than 3 percent by weight as determined by the IP346 method.Procedures for the IP346 method may be found in Standard Methods forAnalysis & Testing of Petroleum and Related Products and BritishStandard 2000 Parts, 2003, 62nd edition, published by the Institute ofPetroleum, United Kingdom.

Suitable TDAE oils are available as Tudalen SX500 from Klaus Dahleke KG,VivaTec 400 and VivaTec 500 from H&R Group, and Enerthene 1849 from BP,and Extensoil 1996 from Repsol. The oils may be available as the oilalone or along with an elastomer in the form of an extended elastomer.

Suitable vegetable oils include, for example, soybean oil, sunflower oiland canola oil which are in the form of esters containing a certaindegree of unsaturation.

The rubber composition optionally includes from 5 to 15 phr of ahydrocarbon traction resin having a glass transition temperature between−40° C. and 100° C. A suitable measurement of Tg for resins is DSCaccording to ASTM D6604 or equivalent. The hydrocarbon resin has asoftening point between 0° C. and 160° C. as determined by ASTM E28which might sometimes be referred to as a ring and ball softening point.

The resin is selected from the group consisting of coumarone-indeneresins, petroleum resins, terpene polymers, styrene-alphamethylstyreneresins, terpene phenol resins, rosin derived resins and copolymersand/or mixtures thereof.

In one embodiment, the resin is a coumarone-indene resin containingcoumarone and indene as the monomer components making up the resinskeleton (main chain). Monomer ingredients other than coumarone andindene which may be incorporated into the skeleton are, for example,methyl coumarone, styrene, alphamethylstyrene, methylindene,vinyltoluene, dicyclopentadiene, cycopentadiene, and diolefins such asisoprene and piperlyene. Suitable coumarone-indene resin is availablecommercially as Novares C30 from Rutgers Novares GmbH.

Suitable petroleum resins include both aromatic and nonaromatic types.Several types of petroleum resins are available. Some resins have a lowdegree of unsaturation and high aromatic content, whereas some arehighly unsaturated and yet some contain no aromatic structure at all.Differences in the resins are largely due to the olefins in thefeedstock from which the resins are derived. Conventional derivatives insuch resins include any C5 species such as cyclopentadiene,dicyclopentadiene, diolefins such as isoprene and piperylene, and any C9species such as vinyltoluene and alphamethylstyrene. Such resins aremade by any mixture formed from C5 and C9 species mentioned above.

In one embodiment, said resin may be a terpene resin comprised ofpolymers of at least one of limonene, alpha pinene and beta pinene.

The styrene/alphamethylstyrene resin is considered herein to be arelatively short chain copolymer of styrene and alphamethylstyrene witha styrene/alphamethylstyrene molar ratio in a range of about 0.05 toabout 1.50. In one aspect, such a resin can be suitably prepared, forexample, by cationic copolymerization of styrene and alphamethylstyrenein a hydrocarbon solvent. Thus, the contemplatedstyrene/alphamethylstyrene resin can be characterized, for example, byits chemical structure, namely, its styrene and alphamethylstyrenecontents and by its glass transition temperature, molecular weight andmolecular weight distribution.

Terpene-phenol resins may be used. Terpene-phenol resins may be derivedby copolymerization of phenolic monomers with terpenes such as limonenesand pinenes.

Resins derived from rosin and derivatives may be used in the presentinvention. Gum and wood rosin have much the same composition, althoughthe amount of the various isomers may vary. They typically contain about10 percent by weight neutral materials, 53 percent by weight resin acidscontaining two double bonds, 13 percent by weight of resin acidscontaining one double bond, 16 percent by weight of completely saturatedresin acids and 2 percent of dehydroabietic acid which contains anaromatic ring but no unsaturation. There are also present about 6percent of oxidized acids. Representative of the diunsaturated acidsinclude abietic acid, levopimaric acid and neoabietic acid.Representative of the monounsaturated acids include dextroplmaris acidand dihydroabietic acid. A representative saturated rosin acid istetrahydroabietic acid. Such resins may be in the form of esters ofrosin acids and polyols such as pentaerythritol or glycol.

In one embodiment, said resin may be partially or fully hydrogenated.

The phrase “rubber or elastomer containing olefinic unsaturation” isintended to include both natural rubber and its various raw and reclaimforms as well as various synthetic rubbers. In the description of thisinvention, the terms “rubber” and “elastomer” may be usedinterchangeably, unless otherwise prescribed. The terms “rubbercomposition,” “compounded rubber” and “rubber compound” are usedinterchangeably to refer to rubber which has been blended or mixed withvarious ingredients and materials, and such terms are well known tothose having skill in the rubber mixing or rubber compounding art.

The vulcanizable rubber composition may include from about 60 to about90 phr of silica.

In one embodiment, the weight ratio of silica to the sum of the amountsof oil and traction resin is greater than 2.2. In one embodiment, theweight ratio of silica to the sum the amounts of oil and traction resinis greater than 3.

The commonly employed siliceous pigments which may be used in the rubbercompound include conventional pyrogenic and precipitated siliceouspigments (silica), although precipitated silicas are preferred. Theconventional siliceous pigments preferably employed in this inventionare precipitated silicas such as, for example, those obtained by theacidification of a soluble silicate, e.g., sodium silicate.

Such conventional silicas might be characterized, for example, by havinga BET surface area, as measured using nitrogen gas, preferably in therange of about 40 to about 600, and more usually in a range of about 50to about 300 square meters per gram. The BET method of measuring surfacearea is described in the Journal of the American Chemical Society,Volume 60, Page 304 (1930).

The conventional silica may also be typically characterized by having adibutylphthalate (DBP) absorption value in a range of about 100 to about400, and more usually about 150 to about 300.

The conventional silica might be expected to have an average ultimateparticle size, for example, in the range of 0.01 to 0.05 micron asdetermined by the electron microscope, although the silica particles maybe even smaller, or possibly larger, in size.

Various commercially available silicas may be used, such as, only forexample herein, and without limitation, silicas commercially availablefrom PPG Industries under the Hi-Sil trademark with designations 210,243, 315 etc.; silicas available from Rhodia, with, for example,designations of Z1165MP and Z165GR and silicas available from Degussa AGwith, for example, designations VN2 and VN3, etc.

Pre-hydrophobated precipitated silica may be used. By pre-hydrophobated,it is meant that the silica is pretreated, i.e., the pre-hydrophobatedprecipitated silica is hydrophobated prior to its addition to the rubbercomposition by treatment with at least one silane. Suitable silanesinclude but are not limited to alkylsilanes, alkoxysilanes,organoalkoxysilyl polysulfides and organomercaptoalkoxysilanes.Alternatively, the precipitated silica may be pre-treated with a silicacoupling agent comprised of, for example, an alkoxyorganomercaptosilaneor combination of alkoxysilane and alkoxyorganomercaptosilane prior toblending the pre-treated silica with the rubber instead of reacting theprecipitated silica with the silica coupling agent in situ within therubber. For example, see U.S. Pat. No. 7,214,731. For variouspre-treated precipitated silicas see, for example, U.S. Pat. Nos.4,704,414, 6,123,762 and 6,573,324. Suitable pre-treated orpre-hydrophobated silica is available commercially for example as Agilon400 from PPG.

The vulcanizable rubber composition may include from about 5 to about 50phr of carbon black.

Commonly employed carbon blacks can be used as a conventional filler.Representative examples of such carbon blacks include N110, N121, N134,N220, N231, N234, N242, N293, N299, S315, N326, N330, M332, N339, N343,N347, N351, N358, N375, N539, N550, N582, N630, N642, N650, N683, N754,N762, N765, N774, N787, N907, N908, N990 and N991. These carbon blackshave iodine absorptions ranging from 9 to 145 g/kg and DBP numberranging from 34 to 150 cm³/100 g.

Other fillers may be used in the rubber composition including, but notlimited to, particulate fillers including ultra high molecular weightpolyethylene (UHMWPE), particulate polymer gels such as those disclosedin U.S. Pat. Nos. 6,242,534; 6,207,757; 6,133,364; 6,372,857; 5,395,891;or 6,127,488, and plasticized starch composite filler such as thatdisclosed in U.S. Patent No. 5,672,639.

It may be preferred to have the rubber composition for use in the tirecomponent to additionally contain a conventional sulfur containingorganosilicon compound. Examples of suitable sulfur containingorganosilicon compounds are of the formula:

Z-Alk-S_(n)-Alk-Z   VIII

in which Z is selected from the group consisting of

where R⁶ is an alkyl group of 1 to 4 carbon atoms, cyclohexyl or phenyl;R⁷ is alkoxy of 1 to 8 carbon atoms, or cycloalkoxy of 5 to 8 carbonatoms; Alk is a divalent hydrocarbon of 1 to 18 carbon atoms and n is aninteger of 2 to 8.

Specific examples of sulfur containing organosilicon compounds which maybe used in accordance with the present invention include:3,3′-bis(trimethoxysilylpropyl) disulfide, 3,3′-bis(triethoxysilylpropyl) disulfide, 3,3′-bis(triethoxysilylpropyl)tetrasulfide, 3,3′-bis(triethoxysilylpropyl) octasulfide,3,3′-bis(trimethoxysilylpropyl) tetrasulfide,2,2′-bis(triethoxysilylethyl) tetrasulfide,3,3′-bis(trimethoxysilylpropyl) trisulfide,3,3′-bis(triethoxysilylpropyl) trisulfide,3,3′-bis(tributoxysilylpropyl) disulfide,3,3′-bis(trimethoxysilylpropyl) hexasulfide,3,3′-bis(trimethoxysilylpropyl) octasulfide,3,3′-bis(trioctoxysilylpropyl) tetrasulfide,3,3′-bis(trihexoxysilylpropyl) disulfide,3,3′-bis(tri-2″-ethylhexoxysilylpropyl) trisulfide,3,3′-bis(triisooctoxysilylpropyl) tetrasulfide,3,3′-bis(tri-t-butoxysilylpropyl) disulfide, 2,2′-bis(methoxy diethoxysilyl ethyl) tetrasulfide, 2,2′-bis(tripropoxysilylethyl) pentasulfide,3,3′-bis(tricyclonexoxysilylpropyl) tetrasulfide,3,3′-bis(tricyclopentoxysilylpropyl) trisulfide,2,2′-bis(tri-2″-methylcyclohexoxysilylethyl) tetrasulfide,bis(trimethoxysilylmethyl) tetrasulfide, 3-methoxy ethoxy propoxysilyl3′-diethoxybutoxy-silylpropyltetrasulfide, 2,2′-bis(dimethylmethoxysilylethyl) disulfide, 2,2′-bis(dimethyl sec.butoxysilylethyl)trisulfide, 3,3′-bis(methyl butylethoxysilylpropyl) tetrasulfide,3,3′-bis(di t-butylmethoxysilylpropyl) tetrasulfide, 2,2′-bis(phenylmethyl methoxysilylethyl) trisulfide, 3,3′-bis(diphenylisopropoxysilylpropyl) tetrasulfide, 3,3′-bis(diphenylcyclohexoxysilylpropyl) disulfide, 3,3′-bis(dimethylethylmercaptosilylpropyl) tetrasulfide, 2,2′-bis(methyldimethoxysilylethyl) trisulfide, 2,2′-bis(methylethoxypropoxysilylethyl) tetrasulfide, 3,3′-bis(diethylmethoxysilylpropyl) tetrasulfide, 3,3′-bis(ethyl di-sec.butoxysilylpropyl) disulfide, 3,3′-bis(propyl diethoxysilylpropyl)disulfide, 3,3′-bis(butyl dimethoxysilylpropyl) trisulfide,3,3′-bis(phenyl dimethoxysilylpropyl) tetrasulfide, 3-phenylethoxybutoxysilyl 3′-trimethoxysilylpropyl tetrasulfide,4,4′-bis(trimethoxysilylbutyl) tetrasulfide,6,6′-bis(triethoxysilylhexyl) tetrasulfide,12,12′-bis(triisopropoxysilyl dodecyl) disulfide,18,18′-bis(trimethoxysilyloctadecyl) tetrasulfide,18,18′-bis(tripropoxysilyloctadecenyl) tetrasulfide,4,4′-bis(trimethoxysilyl-buten-2-yl) tetrasulfide,4,4′-bis(trimethoxysilylcyclohexylene) tetrasulfide,5,5′-bis(dimethoxymethylsilylpentyl) trisulfide,3,3′-bis(trimethoxysilyl-2-methylpropyl) tetrasulfide,3,3′-bis(dimethoxyphenylsilyl-2-methylpropyl) disulfide.

The preferred sulfur containing organosilicon compounds are the3,3′-bis(trimethoxy or triethoxy silylpropyl) sulfides. The mostpreferred compounds are 3,3′-bis(triethoxysilylpropyl) disulfide and3,3′-bis(triethoxysilylpropyl) tetrasulfide. Therefore, as to formulaVIII, preferably Z is

where R⁷ is an alkoxy of 2 to 4 carbon atoms, with 2 carbon atoms beingparticularly preferred; alk is a divalent hydrocarbon of 2 to 4 carbonatoms with 3 carbon atoms being particularly preferred; and n is aninteger of from 2 to 5 with 2 and 4 being particularly preferred.

In another embodiment, suitable sulfur containing organosiliconcompounds include compounds disclosed in U.S. Pat. No. 6,608,125. In oneembodiment, the sulfur containing organosilicon compounds includes3-(octanoylthio)-1-propyltriethoxysilane, CH₃(CH₂)₆C(═O)—S—CH₂CH₂CH₂Si(OCH₂CH₃)₃, which is available commercially as NXTTM fromMomentive Performance Materials.

In another embodiment, suitable sulfur containing organosiliconcompounds include compounds disclosed in U.S. Publication 2006/0041063.In one embodiment, the sulfur containing organosilicon compounds includethe reaction product of hydrocarbon based diol (e.g.,2-methyl-1,3-propanediol) with S-[3-(triethoxysilyl)propyl]thiooctanoate. In one embodiment, the sulfur containing organosiliconcompound is NXT-Z™ from Momentive Performance Materials.

In another embodiment, suitable sulfur containing organosiliconcompounds include those disclosed in U.S. Patent Publication No.2003/0130535. In one embodiment, the sulfur containing organosiliconcompound is Si-363 from Degussa.

The amount of the sulfur containing organosilicon compound of formula Iin a rubber composition will vary depending on the level of otheradditives that are used. Generally speaking, the amount of the compoundof formula I will range from 0.5 to 20 phr. Preferably, the amount willrange from 1 to 10 phr.

It is readily understood by those having skill in the art that therubber composition would be compounded by methods generally known in therubber compounding art, such as mixing the various sulfur-vulcanizableconstituent rubbers with various commonly used additive materials suchas, for example, sulfur donors, curing aids, such as activators andretarders and processing additives, fillers, pigments, fatty acid, zincoxide, waxes, antioxidants and antiozonants and peptizing agents. Asknown to those skilled in the art, depending on the intended use of thesulfur vulcanizable and sulfur-vulcanized material (rubbers), theadditives mentioned above are selected and commonly used in conventionalamounts. Representative examples of sulfur donors include elementalsulfur (free sulfur), an amine disulfide, polymeric polysulfide andsulfur olefin adducts. Preferably, the sulfur-vulcanizing agent iselemental sulfur. The sulfur-vulcanizing agent may be used in an amountranging from 0.5 to 8 phr, with a range of from 1 to 6 phr beingpreferred. Typical amounts of antioxidants comprise about 1 to about 5phr. Representative antioxidants may be, for example,diphenyl-p-phenylenediamine and others, such as, for example, thosedisclosed in The Vanderbilt Rubber Handbook (1978), pages 344 through346. Typical amounts of antiozonants comprise about 1 to 5 phr. Typicalamounts of fatty acids, if used, which can include stearic acid compriseabout 0.5 to about 5 phr. Typical amounts of zinc oxide comprise about 2to about 5 phr. Typical amounts of waxes comprise about 1 to about 5phr. Often microcrystalline waxes are used. Typical amounts of peptizerscomprise about 0.1 to about 1 phr. Typical peptizers may be, forexample, pentachlorothiophenol and dibenzamidodiphenyl disulfide.

Accelerators are used to control the time and/or temperature requiredfor vulcanization and to improve the properties of the vulcanizate. Inone embodiment, a single accelerator system may be used, i.e., primaryaccelerator. The primary accelerator(s) may be used in total amountsranging from about 0.5 to about 4, preferably about 0.8 to about 2.0,phr. In another embodiment, combinations of a primary and a secondaryaccelerator might be used with the secondary accelerator being used insmaller amounts, such as from about 0.05 to about 3 phr, in order toactivate and to improve the properties of the vulcanizate. Combinationsof these accelerators might be expected to produce a synergistic effecton the final properties and are somewhat better than those produced byuse of either accelerator alone. In addition, delayed actionaccelerators may be used which are not affected by normal processingtemperatures but produce a satisfactory cure at ordinary vulcanizationtemperatures. Vulcanization retarders might also be used. Suitable typesof accelerators that may be used in the present invention are amines,disulfides, guanidines, thioureas, thiazoles, thiurams, sulfenamides,dithiocarbamates and xanthates. Preferably, the primary accelerator is asulfenamide. If a second accelerator is used, the secondary acceleratoris preferably a guanidine, dithiocarbamate or thiuram compound.

The rubber composition may be used in a tire, which may be made bymixing (A) from about 60 to about 40 phr of a solution polymerizedstyrene-butadiene rubber having a glass transition temperature (Tg)ranging from −85° C. to −50° C.; (B) from about 40 to about 60 phr ofnatural rubber; (C) from 10 to 40 phr of a process oil; and (D)optionally, from about 5 to 15 phr of a traction resin, and € from 60 to90 phr of silica;wherein the weight ratio of silica to the sums of theamounts of oil and traction resin is greater than 2.2.

The mixing of the rubber composition can be accomplished by methodsknown to those having skill in the rubber mixing art. For example, theingredients are typically mixed in at least two stages, namely, at leastone non-productive stage followed by a productive mix stage. The finalcuratives including sulfur-vulcanizing agents are typically mixed in thefinal stage which is conventionally called the “productive” mix stage inwhich the mixing typically occurs at a temperature, or ultimatetemperature, lower than the mix temperature(s) than the precedingnon-productive mix stage(s). The terms “non-productive” and “productive”mix stages are well known to those having skill in the rubber mixingart. The rubber composition may be subjected to a thermomechanicalmixing step. The thermomechanical mixing step generally comprises amechanical working in a mixer or extruder for a period of time suitablein order to produce a rubber temperature between 140° C. and 190° C. Theappropriate duration of the thermomechanical working varies as afunction of the operating conditions, and the volume and nature of thecomponents. For example, the thermomechanical working may be from 1 to20 minutes.

The rubber composition may be incorporated in a tread of a tire.

The pneumatic tire of the present invention may be a race tire,passenger tire, aircraft tire, agricultural, earthmover, off-the-road,truck tire, and the like. Preferably, the tire is a passenger or trucktire. The tire may also be a radial or bias, with a radial beingpreferred.

Vulcanization of the pneumatic tire of the present invention isgenerally carried out at conventional temperatures ranging from about100° C. to 200° C. Preferably, the vulcanization is conducted attemperatures ranging from about 110° C. to 180° C. Any of the usualvulcanization processes may be used such as heating in a press or mold,heating with superheated steam or hot air. Such tires can be built,shaped, molded and cured by various methods which are known and will bereadily apparent to those having skill in such art.

The following examples are presented for the purposes of illustratingand not limiting the present invention. All parts are parts by weightunless specifically identified otherwise.

EXAMPLE

This example illustrates the advantage of a rubber composition accordingto the invention. Rubber compounds were mixed according to theformulations shown in Table 1, with amounts given in phr. The compoundswere cured and tested for physical properties as shown in Table 2.

Sample C1 is made of a blend of NR and cis-BR, with a silica to oilratio lower than 2.2, showing a good balance of wet, winter and RRrelated properties. In order to further improve properties, cis-BR inSample C1 was exchanged with a functionalized low Tg s-SBR in Samples E2to E4, along with reduced ratio of silica to the sum of oil and resin tolower than 2.2, leading to significant RR, Snow and Wet relatedproperties over Sample C1.

TABLE 1 Composition C1 E2 E3 E4 Styrene-butadiene ¹ 30 30 30 Naturalrubber 70 70 70 70 Cis-polybutadiene ² 30 Naphthenic oil ³ 42 19 10 32Traction resin ⁴ 9 Antioxidant(s) 4 4 4 4 Stearic acid 3 3 3 3 Silane ⁵5.6 4.7 4.7 6.0 Silica ⁶ 90 75 75 Silica ⁷ 75 Carbon Black 2 2 2 2 ZnO2.5 2.5 2.5 2.5 Sulfur 1.6 1.2 1.2 1.2 Accelerator 3.0 3.5 3.5 3.9 ¹Solution polymerized SBR with styrene content of 15% and 1,2-vinylcontent of 30%, Tg = −60° C. obtained from Styron as SLR3402. ² High-cispolybutadiene obtained as BUD1207 from Goodyear Chemical. ³ Naphthenicoil of Tg = −76° C. ⁴ Polyterpene traction resin with a softening point115° C. obtained as Sylvarez TR7115 ⁵ TESPD type silane coupling agent ⁶Hi-Sil 315G-D precipitated silica from PPG with a CTAB surface area of125 m2/g ⁷ 1165MP precipitated silica from Solvay with a CTAB surfacearea of 160 m2/g.

TABLE 2 Composition C1 E2 E3 E4 Dynamic properties (100° C. ) ¹ G' at 1%strain (MPa) 1.7 1.7 1.7 1.5 Tensile properties ² Modulus at 300% strain(MPa) 6.9 9.5 10.2 9.1 Tensile strength (MPa) 14.8 22.0 22.7 23.0Elongation at break (%) 604 585 571 617 Low temperature property ³ G' at1.5% strain, −20° C. (MPa) 6.9 5.0 6.4 5.1 Wet grip property ⁴ Reboundat 0° C. (%) 33.2 33.2 25.8 36.9 RR property ⁴ Rebound at 100° C. (%)62.3 67.3 65.2 67.8 ¹ Data according to Rubber Process Analyzer as RPA2000 instrument by Alpha Technologies, formerly the Flexsys Company andformerly the Monsanto Company. References to an RPA-2000 instrument maybe found in the following publications: H. A. Palowski, et al, RubberWorld, June 1992 and January 1997, as well as Rubber & Plastics News,April 26 and May 10, 1993. ² Data according to Automated Testing Systeminstrument by the Instron Corporation. Such instrument may determineultimate tensile, ultimate elongation, modulii, etc. ³ The G modulus atlow temperatures can be readily be determined by a Metravib TMinstrument at 1.5 percent strain and 7.8 Hertz. The test method isunderstood to be similar to ISO 4664 and DIN 53513. ⁴ Rebound is ameasure of hysteresis of the compound when subject to loading, asmeasured by ASTM D1054. Generally, the lower the measured rebound at 0°C. , the better the wet grip property. Generally, the higher themeasured rebound at 100° C. , the lower the rolling resistance.

While certain representative embodiments and details have been shown forthe purpose of illustrating the subject invention, it will be apparentto those skilled in this art that various changes and modifications canbe made therein without departing from the scope of the subjectinvention.

1. A pneumatic tire having a tread comprising a vulcanizable rubbercomposition comprising, based on 100 parts by weight of elastomer (phr),(A) from about 60 to about 40 phr of a solution polymerizedstyrene-butadiene rubber having a glass transition temperature (Tg)ranging from −85° C. to −50° C.; (B) from about 40 to about 60 phr ofnatural rubber; and (C) from about 5 to 40 phr of a process oil; (D)optionally, from about 5 to 15 phr of a hydrocarbon traction resin (E)from 60 to 90 phr of silica; wherein the weight ratio of silica to thesum of the amounts of oil and traction resin is greater than 2.2.
 2. Thepneumatic tire of claim 1, wherein the solution polymerizedstyrene-butadiene rubber is functionalized with an alkoxysilane groupand at least one functional group selected from the group consisting ofprimary amines and thiols.
 3. The pneumatic tire of claim 1, wherein theoil is selected from the group consisting of aromatic, paraffinic,naphthenic, MES, TDAE, heavy naphthenic oils, and vegetable oils.
 4. Thepneumatic tire of claim 1, wherein the solution polymerizedstyrene-butadiene rubber functionalized with an alkoxysilane group and aprimary amine group, and is represented by the formula (1) or (2)

wherein P is a (co)polymer chain of a conjugated diolefin or aconjugated diolefin and an aromatic vinyl compound, R¹ is an alkylenegroup having 1 to 12 carbon atoms, R² and R³ are each independently analkyl group having 1 to 20 carbon atoms, an allyl group or an arylgroup, n is an integer of 1 or 2, m is an integer of 1 or 2, and k is aninteger of 1 or 2, with the proviso that n+m+k is an integer of 3 or 4,

werein P, R¹, R² and R³ have the same definitions as give for theabove-mentioned formula (1), j is an integer of 1 to 3, and h is aninteger of 1 to 3, with the provision that j+h is an integer of 2 to 4.5. The pneumatic tire of claim 1, wherein the solution polymerizedstyrene-butadiene rubber is functionalized with an alkoxysilane groupand a primary amine group comprises the reaction product of a livingpolymer chain and a terminating agent of the formulaRN—(CH₂)_(x)—Si—(OR′)₃,   I wherein R in combination with the nitrogen(N) atom is a protected amine group which upon appropriatepost-treatment yields a primary amine, R′ represents a group having 1 to18 carbon atoms selected from an alkyl, a cycloalkyl, an allyl, or anaryl; and X is an integer from 1 to
 20. 6. The pneumatic tire of claim 1wherein the solution polymerized styrene-butadiene rubber isfunctionalized with an alkoxysilane group and a thiol, and comprises thereaction product of a living anionic polymer and a silane-sulfidemodifier represented by the formula(R⁴O)_(x)R⁴ _(y)Si—R⁵—S—SiR⁴ ₃ wherein Si is silicon; S is sulfur; O isoxygen; x is an integer selected from 1, 2 and 3; y is an integerselected from 0, 1, and 2; x+y=3; R⁴ is the same or different and is(C₁-C₁₆) alkyl; and R⁵ is aryl, and alkyl aryl, or (C₁-C₁₆) alkyl.
 7. Amethod of making a pneumatic tire, comprising the step of mixing avulcanizable rubber composition comprising, based on 100 parts by weightof elastomer (phr), (A) from about 60 to about 40 phr of a solutionpolymerized styrene-butadiene rubber having a glass transitiontemperature (Tg) ranging from −85° C. to −50° C.; (B) from about 40 toabout 60 phr of natural rubber; and (C) from about 5 to 40 phr of aprocess oil; (D) optionally, from about 5 to 15 phr of a hydrocarbontraction resin (E) from 60 to 90 phr of silica; wherein the weight ratioof silica to the sum of the amounts of oil and traction resin is greaterthan 2.2.